This application claims the benefit of Japanese Patent Application No. 2003-119037, filed on Apr. 23, 2003, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to radio-communication terminals used for data transmission and are capable of determining whether an unauthenticated antenna is connected to the terminal.
2. Description of the Related Art
FIG. 4 is a circuit diagram showing the structure of a known radio-communication terminal. Referring to FIG. 4, an antenna 31 is grounded for direct current through a high-frequency coil 32. The main body of the radio-communication terminal device has a transmission control circuit 35 that has two subsystems: a current detection circuit 33 and a signal output circuit 34. The current detection circuit 33 applies a DC voltage to an antenna connector 36 to determine whether the antenna is connected to the transceiver unit. If the current detection circuit 33 detects a flow of DC current, a transmission signal (high-frequency signal) is supplied from a transmission circuit (not shown) to the antenna connector 36. If the current detection circuit 33 does not detect the DC current, the transmission control circuit 35 determines that the antenna 31 is not connected to the antenna connector 36. The signal output circuit 34 outputs a transmission inhibiting signal, thus preventing the transceiver from transmitting a signal (See for example, Japanese Unexamined Patent Application Publication No. 4-326818).
In the arrangement of the known radio-communication terminal, if the antenna 31 is connected to the antenna connector 36, the transmission control circuit 35 will sense that the antenna 31 is connected to the antenna connector 36 because the current detection circuit 33 detects the DC current, and the transmission circuit will transmit a signal. However, there may be times when the connected antenna 31 does not meet the standards for the transmission circuit because of load balancing factors, or for example, the gain of the antenna 31 may be too large for a given transmitter. If such a radio terminal were allowed to transmit, power levels exceeding those allowed by law may occur and those transmissions may interfere with other receivers and other sensitive electronic devices.